1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus in which a drive force of a motor assists a required value of steering torque applied to a steering wheel operated by an operator of an automotive vehicle.
2. Discussion of the Related Art
An electrically operated power steering apparatus for an automotive vehicle is generally constructed to include (a) a torque transmitting system transmitting steering torque which is applied to a steering wheel by a vehicle operator, to a steerable wheel of the vehicle laying on a road surface, so as to assist the steering torque, (b) a motor applying a drive force thereof to the torque transmitting system, and (c) a controlling device controlling an electric power supply to the motor.
Japanese Patent Publication No. 10-100913 discloses an example of a conventional type of the electrically operated power steering apparatus identified above. In this example, the temperature of a winding of a motor is estimated on the basis of a voltage and a current value of the motor, without the provision of a temperature sensor detecting a temperature of the motor. The estimated temperature results in preventing overheat of the motor.
However, in general, what can be accurately estimated in relation to the temperature of a winding of a motor by the use of a voltage and a current value of the motor is an increase of temperature of the motor winding at each one of a plurality of discrete points of time after a reference point of time. The increase is calculated from the temperature of the heated portion obtained at the reference point of time. The increase is a relative value, not an absolute value of temperature of the motor winding. In addition, the example explained above is not designed to detect or estimate an absolute value of temperature of the motor winding at the reference point of time. Consequently, this example fails to obtain the temperature of the motor with an adequately high precision.
In the case where a temperature sensor is arranged sufficiently near a winding of a motor, the temperature of the motor can be sequentially detected at a high precision. However, in this case, since such a temperature senor is generally expensive, there arises a problem that the substantial increase in the cost of an electrically operated power steering apparatus is unavoidable.
In addition, a problem of generation of heat as a result of a supply of an electric power to a motor arises about the motor in an electrically operated power steering apparatus, but the same problem can also arise about other electrical parts of the apparatus.
It is therefore an object of the present invention to provide an electrically operated power steering apparatus in which the temperature of a heated portion of the apparatus which emits heat as a result of a supply of electric power to a motor can be more precisely obtained in lower cost.
The object may be achieved according to any one of the following modes of this invention. Each of these modes of the invention is numbered like the appended claims, and depends from the other mode or modes, where appropriate. This type of explanation about the present invention is for better understanding of some ones of a plurality of technical features and a plurality of combinations thereof disclosed in this specification, and does not mean that the plurality of technical feature and the plurality of combinations in this specification are interpreted to encompass only the following modes of this invention:
(1) An electrically operated power steering apparatus for an automotive vehicle having a steering wheel to be operated by an operator of the vehicle and a steerable wheel thereof laying on a road surface, comprising:
a torque transmitting system transmitting a steering torque which is applied to the steering wheel by the operator, to the steerable wheels;
a motor applying a drive force thereof to the torque transmitting system so as to assist the steering torque;
a controlling device controlling an electric power supply to the motor, thereby permitting reduction in a required value of the steering torque with the assist of the drive force of the motor; and
a power supply restricting device utilizing a temperature of a heated portion of the electrically operated power steering apparatus which emits heat as a result of the electric power supply to the motor, at a reference point of time, as an initial temperature of the heated portion, utilizing a plurality of electric power-related values each of which is related to at least one of a current and a voltage value of the motor, as a plurality of physical quantities related to temperature increases of the heated portion each of which is an increase of the temperature of the heated portion from that at the reference point of time, and restricting the electric power supply to the motor such that an actual value of the temperature of the heated portion does not exceed a predetermined upper limit thereof.
In general, the temperature of the heated portion, if not changed or gently changed, forms a constant relationship thereof with an ambient temperature of the heated portion. In addition, the ambient temperature of the heated portion can be detected without using a temperature sensor exclusively used for detection of the temperature of the heated portion. Accordingly, the temperature of the heated portion, if not changed or gently changed, can be easily detected or estimated.
The use of at least one of a current and a voltage value of the motor permits estimation of an increase of the temperature of the heated portion from that at a reference point of time. In addition, control of the motor is usually effected with feedback of at least one of a current and a voltage value of the motor. Therefore, a sensor detecting at least one of a current and a voltage value of the motor is usually provided with the electrically operated power steering apparatus. Consequently, in many cases, at least one of a current and a voltage value of the motor can be easily detected.
There exists a technology of using the temperature of the heated portion detected or estimated in the manner mentioned above, as an initial temperature of the heated portion. There also exists a technology of using at least one of a current and a voltage value of the motor as a physical quantity related to an increase of the temperature of the heated portion from that at a time when the initial temperature of the heated portion has been obtained. These two technologies can cooperate with each other to estimate the temperature of the heated portion at each one of a plurality of discrete points of time after the initial temperature of the heated portion has been obtained.
Based on the above findings, the apparatus according to this mode (1) utilizes the temperature of the heated portion at a reference point of time as an initial temperature of the heated portion, and utilizes at least one of a current and a voltage value of the motor as a physical quantity related to an increase of the temperature of the heated portion from that at the reference point of time. Further, the apparatus according to this mode (1) restricts an electric power supply to the motor so as to prevent an actual value of the temperature of the heated portion from exceeding a predetermined upper limit of the temperature of the heated portion.
Consequently, in the apparatus according to this mode (1), it is not indispensable to employ a temperature sensor which exclusively detects the temperature of the heated portion with a high precision and which is expensive. Therefore, a substantial increase in the cost of the apparatus resulting from the addition of a function of obtaining the temperature of the heated portion to an electrically operated power steering apparatus can be easily avoided.
Further, in the apparatus according to this mode (1), a determination as to whether the restriction on the electric power supply to the motor is necessary or not is performed using both of an initial value and a subsequent increase of the temperature of the heated portion from the initial value, both of which are reflected by the actual condition of the heated portion. As a result, the presence of an unnecessary restriction on the electric power supply at lower temperature of the heated portion can be easily avoided, and the absence of a necessary restriction on the electric power supply at higher temperature of the heated portion can also be easily avoided.
The apparatus according to this mode (1) may be adapted to include a temperature sensor capable of precisely detecting the temperature of the heated portion as long as the temperature is substantially in a stable condition thereof. The apparatus according to this mode (1) may be also adapted to include a temperature sensor capable of precisely detecting the temperature of the heated portion not only in a stable condition thereof but also in a transitional condition thereof.
In the apparatus according to this mode (1), the heated portion may be defined as the motor, a switching element connected to the motor and a power supply to the motor, at least one of a plurality of media for transferring current from the power supply to the motor, including such as a wire, a connector, etc., for example. The heated portion also may be defined as at least one of the plurality of media which is especially required to be prevented from being overheated.
In the apparatus according to this mode (1), the torque transmitting system is generally constructed to include (a) a steering shaft rotatable with the steering wheel, (b) an axially movable steering rod permitting the orientation of the steerable wheel to change, and (c) a coupling device operatively coupling the steering shaft and steering rod such that a rotary motion of the steering shaft is converted into a linear motion of the steering rod. In this arrangement, the motor is engaged to at least one of the steering shaft, steering rod and coupling device so as to apply to the at least one of these three elements a drive force of the motor for assisting the steering torque of the steering wheel applied by the vehicle operator.
In the apparatus according to this mode (1), the restriction on the electric power supply may be effected by reducing an actual and absolute value of current (i.e., electric current) of the motor to a certain value. The certain value is smaller than a nominal value of current of the motor available when the restriction on the electric power supply is unnecessary, but is not equal to zero. The restriction on the electric power supply also may be effected by reducing the actual and absolute value to zero.
(2) The apparatus according to the above mode (1), wherein the power supply restricting device comprises:
a temperature estimating means for repeating obtaining one of the plurality of electric power-related values after the reference point of time, for obtaining a sum of the plurality of electric power-related values which have been already obtained, each time a new one of the plurality of electric power-related values has been obtained, the obtained sum being defined as an integrated value of the already obtained plurality of electric power-related values, for estimating the temperature increase of the heated portion on the basis of the obtained integrated value, and for estimating the temperature of the heated portion at each one of a plurality of discrete points of time after the reference point of time, on the basis of the initial temperature and the estimated temperature increase of the heated portion; and
a power supply restricting means for restricting the electric power supply to the motor such that the actual value of the temperature of the heated portion does not exceed the predetermined upper limit, on the basis of the estimated temperature of the heated portion.
In the apparatus according to this mode (2), an increase of the temperature of the heated portion is estimated on the basis of an integrated value of a plurality of electric power-related values. As a result, the temperature increase of the heated portion is estimated by the adequate consideration of a time-dependent change in the electric power-related value. Therefore, in the apparatus according to this mode (2), the precision in estimation of the temperature increase of the heated portion is improved, resulting in another improvement in estimation of the temperature of the heating portion.
(3) The apparatus according to the above mode (2), wherein the power supply restricting means comprises a restricting amount determining means for, when the estimated temperature of the heated portion has reached a reference temperature formulated to be lower than the predetermined upper limit, restricting the electric power supply to the motor, and for repeating determining a restricting amount by which the electric power supply to the motor is to be restricted, on the basis of the estimated temperature of the heated portion at a corresponding one of a plurality of discrete points of time.
In the apparatus according to this mode (3), a restricting amount by which the electric power supply is to be restricted is repeatedly determined after the commencement of restriction on the electric power supply to the motor, on the basis of the temperature of the heated portion estimated at a corresponding one of a plurality of discrete points of time. Therefore, it can surely be avoided that an actual temperature of the heated portion exceeds the predetermined upper limit thereof.
(4) The apparatus according to the above mode (1), wherein the power supply restricting device comprises:
an allowable supply time period determining means for utilizing an initiation point of time of a holding operation of the steering wheel during which the vehicle operator is holding the steering wheel substantially at one steering position thereof which is other than a neutral position thereof, and for determining a time period which is estimated to pass from the initiation point of time of the holding operation until the temperature of the heated portion has reached the reference temperature, on the basis of the initial temperature of the heated portion, a reference temperature of the heated portion at which the restriction on the electric power supply to the motor is to be initiated, and the electric power-related value obtained at the initiation point of time of the holding operation, the determined time period being defined as an allowable supply time period for the electric power supply to the motor; and
a supply restricting means for starting restricting the electric power supply to the motor when the determined allowable time period has passed.
In a holding operation during which the vehicle operator holds the steering wheel at one steering angle thereof, a change in the magnitude of the electric power supply to the motor, i.e., the electric power-related value of the motor is not as large as in a steering operation during which the vehicle operator operates the steering wheel so as to increase a steering angle thereof. Accordingly, if the magnitude of the electric power-related value at an initiation of the holding operation can be recognized, an increase of the temperature of the heated portion at each one of a plurality of discrete points of time after the initiation of the holding operation can be represented as a function of time, during the holding operation.
Based on this finding, in the apparatus according to this mode (4), an allowable power supply time period for the motor is determined as a time period which is estimated to pass from an initiation of a holding operation until the temperature of the heated portion has reached a reference temperature, on the basis of the initial temperature of the heated portion, and the electric power-related value obtained at the initiation of the holding operation. Further, when the determined allowable time period has passed, the restriction on the electric power supply to the motor is initiated.
Therefore, the apparatus according to this mode (4) can easily prevent an actual value of the temperature of the heated portion from exceeding the predetermined upper limit without an indispensable performance of integration of the plurality of electric power-related values.
In the apparatus according to this mode (4), the term xe2x80x9ca holding operationxe2x80x9d may be defined as an operation during which the rate of change in a steering angle of the steering wheel or an amount of change in the steering angel per a certain time period is not larger than a reference value. The term xe2x80x9ca holding operationxe2x80x9d also may be defined as an operation during which the rate of change in the electric power-related value or an amount of change in the electric power-related value is not larger than a reference value.
(5) The apparatus according to the above mode (4), further comprising a second allowable time period determining means for, at a change point of time when a time-dependent change of the electric power-related value occurs, whose amount is not less than a predetermined reference value thereof, during the holding operation, estimating the temperature increase which is an increase of the temperature of the heated portion from that at the initiation point of time of the holding operation, on the basis of an integrated value of the plurality of electric power-related values obtained during a period from the initiation point of time of the holding operation to the change point of time, and for estimating a time period which is expected to pass from the change point of time until the temperature of the heated portion has reached the reference temperature, on the basis of a sum of the estimated temperature increase and the initial temperature of the heated portion, and the electric power-related value obtained at the change point of time, the estimated time period being defined as a second allowable supply time period for the electric power supply to the motor.
There exists a fact that the electric power-related value, i.e., a physical quantity related to the temperature of the heated portion can vary even during a holding operation of the steering wheel by the vehicle operator. There also exists a fact that the temperature of the heated portion when a time-dependent change in the electric power-related value can be estimated on the basis of an integrated value of a plurality of electric-power related values which have been obtained since the initiation of the holding operation, and the initial temperature of the heated portion. In light of these facts, the apparatus according to this mode (5) determines a time period which is estimated to pass from the occurrence of the time-dependent change until the temperature of the heated portion has reached a reference temperature as a second allowable supply time period for the motor.
Accordingly, the apparatus according to this mode (5) can prevent an actual value of the temperature of the heated portion from exceeding the predetermined upper limit thereof due to a time-dependent change in the electric-power-related value during a holding operation of the steering wheel by the vehicle operator.
(6) The apparatus according to the above mode (4) or (5), wherein the allowable time period determining means comprises:
a first means for determining an allowable increase of the temperature of the heated portion on the basis of a difference between the reference temperature and the initial temperature of the heated portion; and
a second means for determining the allowable supply time period corresponding to both the electric power-related value obtained at the initiation point of time of the holding operation and the determined allowable increase of the heated portion, according to a predetermined relationship among the electric power-related value obtained at the initiation point of time of the holding operation, the allowable increase, and the allowable supply time period.
(7) The apparatus according to the above mode (6), wherein the second means comprises a means for determining the allowable supply time period such that the allowable supply time period decreases as the allowable increase decreases, and decreases as the electric power-related value at the initiation point of time of the holding operation increases.
(8) The apparatus according to any one of the above modes (1)-(7), wherein the power supply restricting device comprises an initial temperature determining means for determining an ambient temperature of the heated portion at the reference point of time, as the initial temperature of the heated portion.
(9) The apparatus according to any one of the above modes (1)-(8), further comprising a torque detecting device detecting the steering torque, the torque detecting device including a temperature sensor detecting a temperature of the torque detecting device, the power supply restricting device comprising an initial temperature obtaining means for obtaining the initial temperature of the heated portion on the basis of the temperature detected by the temperature sensor.
In the apparatus according to this mode (9), the same temperature sensor performs both detection of the temperature of the heated portion and acquisition of an initial temperature of the heated portion. Accordingly, the apparatus according to this mode (9) can eliminate the total number of temperature sensors installed in an automotive vehicle, compared with the case where detection of the temperature of the heated portion and acquisition of an initial temperature of the heated portion are separately performed by respective temperature sensors. As a result, the apparatus according to this mode (9) can reduce increase in the cost of the apparatus due to the addition of a function for obtaining the temperature of the heated portion.
(10) The apparatus according to the above mode (9), wherein the temperature sensor is located near the heated portion in the electrically operated power steering apparatus.
(11) The apparatus according to the above mode (9) of (10), wherein the temperature sensor detects the temperature of the torque detecting device as a temperature to be changed according to a substantially constant correlation thereof with an ambient temperature of the heated portion.
(12) The apparatus according to any one of the above modes (9)-(11), wherein the controlling device comprises a means for controlling the electric power supply to the motor on the basis of the steering torque detected by the torque detecting device.
(13) The apparatus according to the above mode (1), wherein the power supply restricting device comprises:
a first allowable supply time period determining means for using an initiation point of time of one continuous steering operation of the steering wheel by the vehicle operator, and for determining a time period which is expected to pass from the initiation point of one continuous steering operation until the temperature of the heated portion has reached a reference temperature at which the restriction on the electric power supply to the motor is to be initiated, on the basis of the initial temperature of the heated portion, a reference temperature formulated to be lower than the predetermined upper limit, and the electric power-related value obtained at the initiation point of time of one continuous steering operation, the determined time period being defined as a first allowable supply time period for the electric power supply to the motor; and
a second allowable supply time period determining means for, at each one of a plurality of discrete points of time after the initiation point of time of one continuous steering operation, estimating the temperature increase which is an increase of the temperature of the heated portion from that at the initiation point of time of one continuous steering operation, on the basis of an integrated value of at least one of the plurality of electrical power-related values which has been obtained since the initiation point of time of one continuous steering operation, and for determining a time period which is expected to pass from each one of the plurality of discrete points of time until the temperature of the heated portion has reached the reference temperature, on the basis of a sum of the estimated temperature increase and the initial temperature of the heated portion, and the electric power-related value obtained at a corresponding one of the plurality of discrete points of time, the determined time period being defined as a second allowable supply time period for the electric power supply to the motor; and
a supply restricting means for starting restricting the power supply to the motor when the first or second allowable supply time period determined by the first or second allowable supply time period determining means has passed.
The apparatus according to this mode (13) can prevent an actual value of the temperature of the heated portion from exceeding the predetermined upper limit, according to the principle corresponding to a principle which is employed in the apparatus according to the above mode (4).
The apparatus according to this mode (13) may be used, irrespective of whether one continuous steering operation set forth in this mode (13) is defined as the holding operation set forth in the above mode (4).